1. Field of the Invention
The present invention relates generally to semiconductor devices, and, more particularly, to semiconductor devices using Group IV semiconductors for light emission upon application of a voltage thereto.
2. Discussion of the Background
Conventional semiconductor light emitting devices are made of chosen materials using Column III-V compound semiconductors including arsenide (As) and phosphorus (P). However, most semiconductor device components for typical use with central processor units (CPUs) and memories are designed to employ silicon (Si), which in turn makes it difficult to monolithically fabricate the semiconductor light emitting devices on a Si substrate along with a CPU and memories. Also, since Si substrates are low in cost, a need exists for development of light emitting elements that are implemented on a Si substrate used as a base plate.
However, Si is an indirect transition type semiconductor which has low light emission efficiency. This results in insufficient light emission in semiconductor light emitting elements with the pn junction of Si simply formed on a Si substrate.
On the contrary, those materials fabricatable on Si substrates include silicon-germanium (SiGe). Although studies are continuing while expecting SiGe to be of the direct transition type, devices with increased luminous efficacy have not yet been developed.
Porous Si filled with pores through etching of the surface of a Si substrate might exhibit strong photo-luminescence light emission at room temperatures. However, the luminous efficiency is extremely low relative to the amount of current injection and is unstable in characteristic. For this reason, porous Si has not been successfully reduced to practice.
Attainability of optically pumped light emission (photoluminescence emission) has been discussed in X. Zhao et al., Japanese Journal of Applied Physics, Vol. 33, L649 (1994). This paper discloses that relatively strong photoluminescence in regions of the spectrum from ultraviolet rays to blue light at room temperatures is observable from Si microcrystals in diameter of 5 nanometers (nm) or less, which were fabricated by heating amorphous Si.
Another report, (Z. H. Lu et al., Nature, Vol. 378, 258 (1995)) shows that relatively strong photoluminescence emission was observed from a super-lattice structure at room temperature. The structure was formed by alternate growth of amorphous Si of several nanometers in thickness and Si oxide films.
Unfortunately, the prior known approaches merely provide device structures with photoluminescence emittability alone. These structures remain incapable of emitting light by current injection and thus lack electrical controllability.
One exemplary light emission device is disclosed in a paper, K. Chen et al., J. Non-Cryst. Solids, 198, 833 (1996). The device disclosed was obtained by forming a multilayer superlattice with a lamination of amorphous Si thin-films and nitrided Si thin-films, heating the lamination for partial recrystallization, and then simply providing electrodes thereon. Two problems are encountered with this approach. The luminous efficacy is low due to increased deviation of size and density of microcrystals because uniform heat application using laser thermal processing is unattainable. Another problem is that the device requires the use of high potential voltages such as 25 volts or above because the multilayer lamination is 200 nm or greater in thickness.
Still another example of a prior art light emittable device is disclosed in Published Japanese Patent Application No. 5-218499 (JP-A-5-218499). This device is designed to provide light emission by locally enhancing the quantum entrapment/confinement effect by use of p-type Si and n-type Si contacted together on the order of several nanometers in size. A further example of prior art is disclosed in JP-A-6-140669, wherein p- and n-type impurity-doped layers with fine lines approximately 10 nm wide or narrower (quantum microlines) are contacted together in a side-by-side manner permitting holes and electrons to be injected from the p- and n-type regions, respectively, for recombination at a junction section leading to emission of light rays (electroluminescence).
The above-referenced prior art approaches are inherently directed to providing the hetero-junction structure with semiconductor microlines directly contacted with other semiconductor layers. A problem associated with this type of structure is that the surrounding semiconductor sections are substantially greater in band gap than the semiconductor sections. This makes it very difficult for electrons and holes to be efficiently injected into the junction section. Even where the electrons and holes might be injected, these can leak to the outside resulting in insufficient luminous efficacy.
As described above, the prior art Si-base semiconductor light emitting devices do not achieve sufficient efficiency of light emission.